I'm with @Enginseer that if you want CS spies and major civ spies be balanced with each other, the easiest thing would be simplifying major civ spying to have automatic missions (= CS rigging in strength) + active button (= coups in strength). So I've come up with the following proposals:
Base number = 1 + Number of CS at game start / 10
All spy gains are multiplied by this base number, except the initial English spy.
Shadow Networks
+3% culture per active spy, divided by base number. Still capped at 30%, but it's not like you can reach there.
Spy rank: Recruit = 1, Agent = 2, Special Agent = 3, not affected by England UA
Spy level = spy rank for non-England civs, and spy rank + 1 for England
Travel time
Travel time to any city is simplified: 1 turn if target city is
connected to your capital (need Open Borders if other major civ, or Friendship if CS), and 2 turns if not. This is determined when the spy is sent out and won't change even if connection is gained/lost while it's traveling.
Establish surveillance
Foreign city: base 5 turns, -1 turn per spy level above 1 and -1 turn if Popular to target civ.
Own city: instant
Currently, there are two types of function calls to level up a spy.
By the way, uncovering intrigue for "building up an army for a naval attack" currently uses the old method of having 15% chance to level up the spy. This is likely an oversight when switching to new experience model and should be changed to +25 experience instead like the land attack version.
List of automatic missions (names not final, need suggestions):
Steal Food: -50%
Food per turn in the target city, and the lost amount is added to your 
Capital. This does not affect Distress calculation.
Steal Gold: -50%
Gold per turn in the target city, and the lost amount if added to your treasury. This does not affect Poverty calculation.
Steal Research Materials: -50%
Science per turn in the target city, and the lost amount is added to your research progress. This does not affect Illiteracy calculation.
Spread Heresy: -50%
Faith per turn in the target city, and the lost amount is added to your global Faith pool. Also -50% Pressure emitted by the target city (this is especially useful against India who relies on high base pressure).
Kidnap Specialists (against Capitals only): -1
GPP per Specialist in the target city, and +1 GPP per Specialist in your Capital (stackable).
All numbers above are diminished by half of the target city's security level (will be shown on the notification when the mission starts). Stolen yields are calculated based on the base yield, pre-modifiers. For example, if the target city has 100 +60%, security level is at 30% and the Steal Research Materials mission is active, the siphoned amount is 100 * 50% * (1 - 30% / 2) = 42.5 . This amount is recalculated per turn using the current city yield, but keeping the security level at the start of mission.
Automatic missions last for up to 15 turns as long as the spy stays in the city. The target is notified when a mission starts or ends (some spy is stealing X yield in the city Y). At the end of the 15 turns, you get another event popup and can pick a mission again.
Detailed explanations of what happens on the "election" turn (it's basically just like that):
When a spy has finished establishing surveillance, they enter the "Gathering Intel" state and starts gaining "intel" points equal to (spy level)^2 per turn. Spies gain double the amount if owner has the Covert Action tenet. Counterspies gain double the amount if owner has the Police State tenet.
On election turn, all intel points are used as weights to randomly determine the "winner". The winner in this case gets to carry out their automatic mission. Intel points are reset to 0 for the winner.
List of tactical missions (names not final, need suggestions):
Formula for success rate
B = Base success rate, differs from quest to quest
C = Counterspy level, 0 if not present
D = Spy level difference (your spy - counterspy). Target spy level is counted as 0 if there's no counterspy.
P = Policy Modifier, 1 if having Covert Action, 0 otherwise
N = Number of times your spy has attempted an automatic mission. This count is reset when the spy leaves the city.
S = Security level, ranged from 0% to 100%
O = Open Borders Modifier, 20% if target's borders are (peacefully) open to you, 0 otherwise
T = Tech/Policy difference (theirs - yours). It compares the tech count and policy count only, and doesn't factor in tech/policy choices. Bottomed at 0 for each category, i.e. T = max(0, tech difference) + max(0, policy difference)
Modded base success rate M = B * 3 / (C + 3) + 20% * N * (D + 4) / 4 if D is positive (or 0), and B * 3 / (C + 3) + 20% * N * 4 / (D + 4) if D is negative
Exponent E (aka number of rolls) = 1 + P + O + 0.1T - S
Final success rate = 1 - (1 - M) ^ E, similar to how coup success rate is calculated.
This means you can only ensure a 100% success rate through keeping the spy in the city for a long time. Using a high level spy and facing off a low level counterspy (or no counterspy) speeds up this process. Other modifiers merely increase the number of rolls. The defending side should make sure to send a counterspy if they notice the city keeps being hit by automatic missions!
The Formula
Base security level = 50%
Negative:
in the city and no foreign auto mission effect occurs for the next 15 turns, and the following effects occur depending on spy level and buildings in the city/empire:
Constabulary built in the city:
Counterspy owner gains 25
and 25 , scaling with era, and identifies (+kicks out) all foreign spies.
Counterspy level 2 or above + Police Station built in the city or White Tower built in any owned city:
All foreign spies are captured. For each captured spy, you can pick one of the options below via event popups.
(X = rank of the captured spy)
Counterspies also have their own tactical mission: Capture Foreign Spies, that's available only if the counterspy is level 3 or above.
Attempt to capture all foreign spies immediately. For each foreign spy, the below outcomes can happen:
1. The spy is captured. This triggers the above event from capturing spies.
2. The spy is killed. This does not identify the spy, but the owner loses it.
3. The spy escapes and goes back to the hideout.
Formula for success rate
D = Spy level difference (counterspy - spy)
P = Policy Modifier, 1 if having Double Agent, 0 otherwise
N = Number of times your counterspy has attempted an automatic mission. This count is reset when the counterspy leaves the city.
S = Security level, ranged from 0% to 100%
Base success rate B = 60%
Modded base success rate M = B + 20% * N * (D + 4) / 4 if D is positive (or 0), and B + 20% * N * 4 / (D + 4) if D is negative
Probability of Outcome 1 (P1) = (1 - (1 - M) ^ (P + S - 1) / 2
Probability of Outcome 2 (P2) = (1 - (1 - M) ^ (S - 1)) / 2
Probability of Outcome 3 (P3) = 1 - P1 - P2
After doing this mission, the counterspy immediately goes into retirement and a new spy replaces it. You should make sure there are a lot of foreign spies acting in the city before attempting this.
Current experience amounts:
Major city spying: +20 for successful mission (identified or not)
Uncovering intrigue: +25 except the oddity mentioned in Proposal 3
Rigging election: +15 if successful
Coup: +50 if successful
Killing a foreign spy: +50
Proposed experience amounts:
Automatic mission (spy): on election turn, +20 if successfully carried out mission, +10 if not
Automatic mission (counterspy): on election turn, +10 experience, and +20 additional for each foreign spy identified/captured
Tactical mission (spy): +50 if successful
Tactical mission (counterspy): N/A (retirement)
Diplomat: +1 every turn
Uncovering intrigue: +15
Rigging election: +20 if successful
Coup: +50 if successful
Covert Action: Success rate for automatic missions doubled, and success rate for tactical missions increased (+1 roll). Receive 1 Spy (plus 1 Spy for every 10 City-States ever existed)
Double Agents: Can access the "Turn spy into double agent" option if a foreign Spy is captured. Counterspies are more likely to capture foreign Spies in the Capture Foreign Spies mission (+1 roll). Receive 3 Spies (plus 3 Spies for every 10 City-States ever existed).
Police State: Can access the "Make effective propaganda with spy" option if a foreign Spy is captured. Counterspies are twice as likely to succeed in automatic missions. +3 Happiness from Courthouses, and +1 Happiness from Police Stations. +100% 
towards Courthouses.
+25% Security Level
-1
Unhappiness from Distress
Unlocks "ransom" option if foreign spy is captured in this city
Counterspy in this city can automatically identify foreign spies
+25 Science and Culture in this city for each successful counterspy automatic mission, scaling with era
Wat
All Constabulary stuff
+33% Security Level instead of +25%
Otherwise same as current version
Police Station
+25% Security Level
+1% Security Level per
Population
-1 Unhappiness from Distress, Poverty, Illiteracy and Boredom
Unlocks "gather intel" option if foreign spy is captured in this city
Counterspy in this city can automatically capture foreign spies
White Tower
All Ironworks stuff
+3
+2 (in addition to the Production from Ironworks)
+25 Gold when a building is built, scaling with era (in addition to the Science from Ironworks)
Provides 1 free
Great Work of Art
+25 Gold and Science in this city for each successful automatic or tactical mission, scaling with era
Constabulary, Castle, Armory: +1 Culture
Feel free to voice out your opinions here or on Discord.
Proposal 1
Number of spiesBase number = 1 + Number of CS at game start / 10
All spy gains are multiplied by this base number, except the initial English spy.
Shadow Networks
+3% culture per active spy, divided by base number. Still capped at 30%, but it's not like you can reach there.
Proposal 2
All duration mentioned below does NOT scale with game speed.Spy rank: Recruit = 1, Agent = 2, Special Agent = 3, not affected by England UA
Spy level = spy rank for non-England civs, and spy rank + 1 for England
Travel time
Travel time to any city is simplified: 1 turn if target city is
connected to your capital (need Open Borders if other major civ, or Friendship if CS), and 2 turns if not. This is determined when the spy is sent out and won't change even if connection is gained/lost while it's traveling.Establish surveillance
Foreign city: base 5 turns, -1 turn per spy level above 1 and -1 turn if Popular to target civ.
Own city: instant
Proposal 3
Spy ExperienceCurrently, there are two types of function calls to level up a spy.
- LevelUpSpy(uiSpyIndex) simply raises the rank of the spy. Experience is retained. Only used in one situation in VP (likely unintended, see below).
- LevelUpSpy(uiSpyIndex, iExperience) adds experience to the spy, and ranks it up if it has 100 experience or more. All excess experience is discarded when a spy levels up this way.
By the way, uncovering intrigue for "building up an army for a naval attack" currently uses the old method of having 15% chance to level up the spy. This is likely an oversight when switching to new experience model and should be changed to +25 experience instead like the land attack version.
Proposal 4
Automatic missions
When your spy finishes establishing surveillance, you get vision around the city (same as CS) and a list of "automatic" missions to pick from. Every 15 turns (same as election rigging time), all foreign spies in the city would fight it out (using the same mechanics as rigging) to determine which spy gets to do their mission for the next 15 turns. No spy deaths or identifications occur here unless a counterspy is present (more on that below).List of automatic missions (names not final, need suggestions):
Steal Food: -50%
Food per turn in the target city, and the lost amount is added to your Capital. This does not affect Distress calculation.Steal Gold: -50%
Gold per turn in the target city, and the lost amount if added to your treasury. This does not affect Poverty calculation.Steal Research Materials: -50%
Science per turn in the target city, and the lost amount is added to your research progress. This does not affect Illiteracy calculation.Spread Heresy: -50%
Faith per turn in the target city, and the lost amount is added to your global Faith pool. Also -50% Pressure emitted by the target city (this is especially useful against India who relies on high base pressure).Kidnap Specialists (against Capitals only): -1
GPP per Specialist in the target city, and +1 GPP per Specialist in your Capital (stackable).All numbers above are diminished by half of the target city's security level (will be shown on the notification when the mission starts). Stolen yields are calculated based on the base yield, pre-modifiers. For example, if the target city has 100
+60%, security level is at 30% and the Steal Research Materials mission is active, the siphoned amount is 100 * 50% * (1 - 30% / 2) = 42.5 . This amount is recalculated per turn using the current city yield, but keeping the security level at the start of mission.Automatic missions last for up to 15 turns as long as the spy stays in the city. The target is notified when a mission starts or ends (some spy is stealing X yield in the city Y). At the end of the 15 turns, you get another event popup and can pick a mission again.
Detailed explanations of what happens on the "election" turn (it's basically just like that):
When a spy has finished establishing surveillance, they enter the "Gathering Intel" state and starts gaining "intel" points equal to (spy level)^2 per turn. Spies gain double the amount if owner has the Covert Action tenet. Counterspies gain double the amount if owner has the Police State tenet.
On election turn, all intel points are used as weights to randomly determine the "winner". The winner in this case gets to carry out their automatic mission. Intel points are reset to 0 for the winner.
Tactical missions
The coup equivalent are the "tactical" missions, which your spy can attempt at any time after surveillance is established. Pressing the button would bring up a list of tactical missions, each with a different base success rate. Final success rate is determined by spy level, counterspy level, policy, security level, open borders, tech/policy count difference and number of times the spy has carried out automatic missions in this city, just like how coup rate is calculated. If the spy fails, it dies. Either way it's identified and triggers a diplomatic penalty. The target city cannot be hit by another tactical mission for 10 turns.List of tactical missions (names not final, need suggestions):
| Name | Prerequisite | Effect | Base success rate |
|---|---|---|---|
| Steal a tech | Target must have at least one tech you haven't researched. | Gain one tech the target has researched that you haven't. | 50% |
| Steal a Great Work | Target city must have at least one Great Work. | Steal a random Great Work in the target city. | 50% |
| Arm the local populace | Target city must be unhappy. | Target city enters resistance for 2 turns. 5 Rebel units spawn around the city. No  Tourism is generated in the city for 10 turns. | 50% |
| Sabotage rural infrastructure | Spy must be level 2 or above. Target city must have at least one pillageable improvement on a resource tile that's not a Fort or Citadel. | All improved resource tiles owned by this city are pillaged. Each road/railroad also has a 50% chance to be pillaged. Does not affect Forts and Citadels. | 40% |
| Target city defense | Spy must be level 2 or above. | All Forts and Citadels owned by this city are pillaged. City is blockaded, cannot perform  ranged strike or benefit from extra strength from buildings for 3 turns. | 40% |
| Sabotage city production | Spy must be level 2 or above. Target city must be producing something (not Process). | Completely remove all progress towards the current production. Reduced production is capped at 10 turns of current city production. It does not undo investments. | 60% |
Formula for success rate
B = Base success rate, differs from quest to quest
C = Counterspy level, 0 if not present
D = Spy level difference (your spy - counterspy). Target spy level is counted as 0 if there's no counterspy.
P = Policy Modifier, 1 if having Covert Action, 0 otherwise
N = Number of times your spy has attempted an automatic mission. This count is reset when the spy leaves the city.
S = Security level, ranged from 0% to 100%
O = Open Borders Modifier, 20% if target's borders are (peacefully) open to you, 0 otherwise
T = Tech/Policy difference (theirs - yours). It compares the tech count and policy count only, and doesn't factor in tech/policy choices. Bottomed at 0 for each category, i.e. T = max(0, tech difference) + max(0, policy difference)
Modded base success rate M = B * 3 / (C + 3) + 20% * N * (D + 4) / 4 if D is positive (or 0), and B * 3 / (C + 3) + 20% * N * 4 / (D + 4) if D is negative
Exponent E (aka number of rolls) = 1 + P + O + 0.1T - S
Final success rate = 1 - (1 - M) ^ E, similar to how coup success rate is calculated.
This means you can only ensure a 100% success rate through keeping the spy in the city for a long time. Using a high level spy and facing off a low level counterspy (or no counterspy) speeds up this process. Other modifiers merely increase the number of rolls. The defending side should make sure to send a counterspy if they notice the city keeps being hit by automatic missions!
Security level
Security level and spy resistance are merged back into one thing (no more sliding mechanism), determined by traits, buildings, policies, population, trade routes, public works, local happiness. Counterspy is NOT accounted into this to not reveal its presence. It's shown as the star rating on the espionage screen.The Formula
Base security level = 50%
Negative:
- -4% per city population
- -8% per excess unhappiness
- -2% per
trade route from/to this city
- +1000% before anyone reaches Renaissance or everyone has gained their first spy, whichever is easier to implement
- +25% for England
- +25% from Empiricism policy
- +15% from Bletchley Park (global)
- +10% per Public Work
- +25% from Constabulary
- +33% from Wat
- +25% from Police Station
- +1% per city population if Police Station is built
- +10% from Military Base
- +15% from National Intelligence Agency
- +100% from Great Firewall
- +25% from Piazza San Marco
Counterspying
Counterspies can also fight for automatic missions every 15 turns with a bonus (mentioned above). If a counterspy wins this, +1 in the city and no foreign auto mission effect occurs for the next 15 turns, and the following effects occur depending on spy level and buildings in the city/empire:Constabulary built in the city:
Counterspy owner gains 25
and 25 , scaling with era, and identifies (+kicks out) all foreign spies.Counterspy level 2 or above + Police Station built in the city or White Tower built in any owned city:
All foreign spies are captured. For each captured spy, you can pick one of the options below via event popups.
(X = rank of the captured spy)
| Event choice | Prerequisite | Description |
|---|---|---|
| Release spy | None | The captured spy returns to hideout. |
| Imprison spy | None | The captured spy is unavailable for use for 5X turns. Triggers a negative opinion with the spy owner. |
| Release spy after ransom paid | Constabulary built in city. Spy owner must have the gold to pay. | You gain 50X Gold while the spy owner loses the same. Amount scales with era. The spy owner is notified of this. |
| Release spy after intel gathered | Police Station built in city. | You gain 50X Science, scaling with era. The spy owner is notified of this. |
| Turn spy into double agent | Double Agent tenet, you must have the gold to spend. | Spend 100X , scaling with era. For the next 15 turns, you gain Science as if the captured spy is doing the "Steal Research Materials" automatic mission in the spy owner's capital. The spy owner does not lose Science and is NOT notified of this, and instead gets a notification as if the spy is released. |
| Make effective propaganda with spy | Police State tenet | +5 Happiness in the city and +15% on all military units for 5X turns (the duration stacks). The captured spy is unavailable for use for 5X turns. Triggers a negative opinion with the spy owner. |
Counterspies also have their own tactical mission: Capture Foreign Spies, that's available only if the counterspy is level 3 or above.
Attempt to capture all foreign spies immediately. For each foreign spy, the below outcomes can happen:
1. The spy is captured. This triggers the above event from capturing spies.
2. The spy is killed. This does not identify the spy, but the owner loses it.
3. The spy escapes and goes back to the hideout.
Formula for success rate
D = Spy level difference (counterspy - spy)
P = Policy Modifier, 1 if having Double Agent, 0 otherwise
N = Number of times your counterspy has attempted an automatic mission. This count is reset when the counterspy leaves the city.
S = Security level, ranged from 0% to 100%
Base success rate B = 60%
Modded base success rate M = B + 20% * N * (D + 4) / 4 if D is positive (or 0), and B + 20% * N * 4 / (D + 4) if D is negative
Probability of Outcome 1 (P1) = (1 - (1 - M) ^ (P + S - 1) / 2
Probability of Outcome 2 (P2) = (1 - (1 - M) ^ (S - 1)) / 2
Probability of Outcome 3 (P3) = 1 - P1 - P2
After doing this mission, the counterspy immediately goes into retirement and a new spy replaces it. You should make sure there are a lot of foreign spies acting in the city before attempting this.
Spy experience in the new model
This should not have conflict with Proposal 3, which changes the LevelUpSpy function only.Current experience amounts:
Major city spying: +20 for successful mission (identified or not)
Uncovering intrigue: +25 except the oddity mentioned in Proposal 3
Rigging election: +15 if successful
Coup: +50 if successful
Killing a foreign spy: +50
Proposed experience amounts:
Automatic mission (spy): on election turn, +20 if successfully carried out mission, +10 if not
Automatic mission (counterspy): on election turn, +10 experience, and +20 additional for each foreign spy identified/captured
Tactical mission (spy): +50 if successful
Tactical mission (counterspy): N/A (retirement)
Diplomat: +1 every turn
Uncovering intrigue: +15
Rigging election: +20 if successful
Coup: +50 if successful
Tenet changes
From now on, the term "automatic mission" includes election rigging and "tactical mission" includes coup.Covert Action: Success rate for automatic missions doubled, and success rate for tactical missions increased (+1 roll). Receive 1 Spy (plus 1 Spy for every 10 City-States ever existed)
Double Agents: Can access the "Turn spy into double agent" option if a foreign Spy is captured. Counterspies are more likely to capture foreign Spies in the Capture Foreign Spies mission (+1 roll). Receive 3 Spies (plus 3 Spies for every 10 City-States ever existed).
Police State: Can access the "Make effective propaganda with spy" option if a foreign Spy is captured. Counterspies are twice as likely to succeed in automatic missions. +3
Happiness from Courthouses, and +1 Happiness from Police Stations. +100% towards Courthouses.Building changes
Constabulary+25% Security Level
-1
Unhappiness from DistressUnlocks "ransom" option if foreign spy is captured in this city
Counterspy in this city can automatically identify foreign spies
+25
Science and Culture in this city for each successful counterspy automatic mission, scaling with eraWat
All Constabulary stuff
+33% Security Level instead of +25%
Otherwise same as current version
Police Station
+25% Security Level
+1% Security Level per
Population-1
Unhappiness from Distress, Poverty, Illiteracy and BoredomUnlocks "gather intel" option if foreign spy is captured in this city
Counterspy in this city can automatically capture foreign spies
White Tower
All Ironworks stuff
+3
+2
(in addition to the Production from Ironworks)+25
Gold when a building is built, scaling with era (in addition to the Science from Ironworks)Provides 1 free
Great Work of Art+25
Gold and Science in this city for each successful automatic or tactical mission, scaling with eraConstabulary, Castle, Armory: +1
CultureFeel free to voice out your opinions here or on Discord.
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